Fragmented mycelia, washed mycelial pellets and cell free extracts of Penicillium rubrum Stoll will be inoculated into appropriately diluted sterile glucose-mineral salts broth containing 3H- or 14C- labeled radioactive materials. After incubation for designated periods, under specified cultural conditions, rubratoxin will be recovered by solvent extraction and determined spectrophotometrically. Radioactive rubratoxin will then be introduced into mammalian tissue culture cells and small mammals (mice/rats) and metabolites will be recovered and tested for biological activity with particular reference to suppression of RNA polymerase activity in the rat or mouse, alteration of RNA metabolism in Euglena or binding to polyribosomes in hetero-ploid lung cells or monkey kidney cells. Immunosuppression activity or rubratoxin or its metabolites will be determined by binding these materials to appropriate haptens, injecting them into rabbits, and recovering the gamma globulins by immunoelectrophoresis. Their ability to control rejection of transplanted organs will be evaluated by comparison with authentic antithymocyte globulins (ATG). Later procedure will be done in collaboration with investigators at the Medical College of Georgia in Augusta, Georgia.